Microwave phase compensation system



oct. 22, 1957 2,810,908

c. F. CRAWFORD ET A1.

MICROWAVE PHASE COMPENSATIQN SYSTEM Filed oet. 1o, 1951 ATTORNEY Unid ,am

MICROWAVE PHASE coMPENsATIoN SYSTEM Carl Francis Crawford, Pennsauken, N. J., and John Randpiph Ford, Narberth, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application october 10, 19.51, serial No. 250,726 s claims. (ci. 34a-7st) This invention relates to microwave guide transmission systems that involve in their operation the use of phase relationships in two wave guide components of the system, and particularly to a pair of waveguides having a relative phase shift maintained substantially constant over a wide frequency bandrabout an operating frequency.

In one conventional simultaneous lobing tracking radar system, for example, energy from an oscillating source is fed to a simultaneous lobing horn fed antenna array through a single wave guide. Adjacent to and connected to a second horn of the antenna array is a second Wave guide that is coupled by a directional slot coupler to the irst guide. To compensate for the 90 degree shift in phase occurring at the 4slot coupling, a 90 degree phase shifter` is inserted in the first guide between the slot and the antenna feed. As a result, energy is radiated simultaneously from the antenna array in a pencil beam pattern. Y

When the reflected pulses are received back by the antenna array, the received energies pass from the separate feed horns through theirtwo transmission guides until they reach the slot coupler, where the energy in each guide divides and part of the energy in each guide passes into the other guide. By suitably combining these outputs, sum and difference outputs can be formed. The difference output amplitude is proportional to the absolute error between the bearings of the target and the antenna while the sum output amplitude is relatively insensitive to this error. By comparing the sum and difference outputs in a phase detector, a D.-C. voltage can be obtained that has directional sense and which is proportional to the error.

One method of maintaining the desired phase relationships of a pair of waveguides over a broad band, is by constructing one waveguide longer than the other and making the waveguides of different cross-sectional dimensions thereby to change the phase velocity of one with respect to the other. Further, by'suitably selecting the dimensions so that the product of the waveguide length and the propagation wavelength in the waveguide for one waveguide is equal to the like product for the other waveguide, as taught inthe copending application of Nathaniel I. Korman, Ser. No. 165,016 led May 29, 1950, entitled Microwave Phase Compensation System, a broadband effect is secured. As a result of this broadband effect, a desired change in phase relationship between two Waveguides (e. g. 90) may be maintained over a broad band of frequencies. However, to get different relative phase, shifts, the two waveguides must be of different lengths. As both the entry points of the waveguides are at one place and the exit points at another, at least one bent waveguide is required. But bent waveguides are often diiiicult to construct.

It is an object'y of the present invention to provide a l being insensitive to frequency changes, or broad banded,

at the operating frequency.

2,810,908 Patented Oct. 22, 157

It is a further object of the invention to provide a novel arrangement of a pair of waveguides in which the phase shift over one may differ from that over the other by a predetermined amount which is substantially independent of frequency at or near a known mean operating frequency.

lt is another object of the invention to provide, such an arrangement with a pair of waveguides of equal physical length, and which may therefore be straight and have the same energy entrance and exit points.

It is another object of the invention to improve simultaneous lobing radar systems, particularly by providing an improved pair of waveguides which may be of the same length, and yet afford a desiredrelative phaseV shift over an especially broad band of frequencies near the operating frequency.

The result of such selection is that the relative phaseV shift of energy between the two waveguides is the same over a broad band of frequencies in the region of the operating frequency. The said selection of dielectrics and i dimensions is made so that the product of the dielectric constant and propagation wavelength in the waveguide at the operating Vfrequency for one waveguide is equal to the like product for the other waveguide. `It may then be shown that the rates of change of phase shift for energy passing through the waveguides are equal, at the operating frequency. The desired broad banded effectV is thereby achieved. f

The foregoing and other objects, advantages, and novel features of the invention will be more apparent from the following 'description when taken in connection with the of hollow pipe rectangular waveguides 14, 16.

` phase shifted outputs are coupled in a phase shifting accompanying drawing in which:

Fig. 1 is a longitudinal cross-sectional view of a preferred embodiment of the invention employing air dielectric in one of a pair of waveguides, and Y Fig. 2 is a transverse cross-sectional View of another embodiment of the invention.

Referring to Fig. 1, a portion of a simultaneous lobing radar system is shown comprising a pair of-horn antennas 10, 12, supplying energy respectively from or to a pair The waveguide 14 may be filled with a.diele'ctric 18. vThe remainder of the system may be Vthe same as that shown and described in the copending application of W. P. Bollinger et al., Serial No. 198,272, filed November 30, 1950. Briey, in the system as explained in the said Bollinger et al. application, signals from the two-horn antenna are applied respectively to the two waveguides 14, 16. In one waveguide 416 there is a phase advance of 90 relative to the otherV waveguide 14. The two directional coupler 34, and two outputs, one a vector sum voltage and the other a vector difference voltage are applied respectively to a sum amplifier 30 and a difference amplifier 32. -These amplifiers provide two ont-Y puts, that from sum amplifier 30 proportionall to the amplitude of the echo from an echoingV object; and that from difference amplifier 32 proportional insign and amplitude to the angular'displacement of the echoing object from the optical axis of the system as a whole.` f The echoes may arise from energy applied from a transmitter 3S to a suitable transmit-receive switch arrangement 36 interposed between the coupler 34 and the sum amplifier 30. If the switch is properly located in a manner which is known to those skilled in the art, pulsed energy on transmission will fire the switch and pass to the antennas. After transmission ceases and on reception, echo energy from objects reflecting the transmitted energy will pass uninterrupted to the sum and difference amplifiers as heretofore explained. These two outputs may be used in various known ways, for example as explained and described in the said Bollinger et al. application. It is not necessary to describe the system further to explain the invention. The portions of the system in the Bollinger et al. application which may be replaced by the portion of the system here shown will be obvious to those skilled in the art. The description of the invention will make apparent to those skilled in the art the use of the invention in this system or in others.

The waveguides 14, 16 are of different electrical lengths but the same physical length, and may therefore conveniently be placed side by side and have a common wall such'as wall 21, if desired. In the example shown here, the electrical lengths are to differ by 90 electrical degrees at the operating frequency. It is desirable to preserve this relative phase shift between energy passing through waveguides 14 and 16 over a wide frequency band about the operating frequency.

Let the dielectric constant of dielectric 18 be el, and the dielectric constant of the dielectric in the other Waveguide be e2. Let the wavelengths in the waveguides 14, 16 at the operating frequency be respectively Agp and Aga, and the waveguide widths be al and aY2 respectively. The phase shift in a length l of waveguide .for waveguide 14 is:

where l is the length of waveguide, A is the free space wavelength at the operating frequency, and a1 is the width of the rectangular waveguide 14. It is assumed that the waveguides are operated in the dominant and usual TEo, 1 mode.

The wavelength in waveguide 14 is:

l-xxi- M3 Therefore, the phase shift of energy at an operating frequency in waveguide 14 remains relatively the same as that for energy of the same frequency in waveguide 16 when E1g1=e2hg2 (4) The relation (4) is that the product of the dielectric constant and the waveguide wavelength at the operating frequency in one waveguide is equal to that product for the other waveguide. When this relation holds for waveguides of equal physical length, their electrical lengths will have the same difference over a broad band of frequencies about the operating frequency.

An especially desirable embodiment can be formed with both waveguides of the same internal dimensions,v r

but different dielectric media, one being air filled. Suppose then that a,=a2=a, and that one wevaguide 16 has air dielectric.

Then we desire to have because s2-:1, substantially, waveguide 16 having air or vacuum as the dielectric medium.

It may be proved that it is sufficient in this case if whereupon (5) is satisfied and one waveguide may have air dielectric and a1=a2.

To satisfy Equation 6 however, requires that a certain relationship exist between the dielectric constant e1 of waveguide 14 and the waveguide widths a.

The differential phase shift between the points of energy input and output of the equal lengths of waveguide is 2f/rl 21rl 2 z a02-2112 1s-M( el n-Mzug/@ai awning/VLM 7) when waveguide 16 is air filled. Since hollow-pipe waveguides are manufactured in standard widths, the value of a to be inserted in Equations 6 and 7 is preferably that usually selected for the operating frequency. However, the length l and the dielectric constant e may still be selected to satisfy the desired criteria, because it turns out that in most practical cases the dielectric constant required is less than is available in several low loss materials. Hence the required value of e is readily realized by filling only a portion of the waveguide with a single dielectric or using two dielectrics as more fully described in connection with Fig. 2. Hence Equations 6 and 7 can always be satisfied. There is, however, one additional criteria in a practical design that may make it necessary to use a non-standard width a, in order to broadband at a Agiven wavelength xo. In order to avoid refiections in the loaded guide, the loaded section should preferably be exactly an integral number of half guide wavelengths long at the center operating frequency. If this is done the reflection arising at the beginning of the section cancels out the refiection arising at the end of the section.

It may at times be desirable to use different solid di electric media in waveguides 14tand 16. Various values of e, and e2 can be obtained, if .not directly, by completely filling the waveguide, then by partially filling the waveguide, for example, as illustrated by the dielectric 20 partially filling waveguide 14 shown in Fig. 2; or by using two different solid dielectrics in one waveguide, as the dielectrics 22 and 24 in waveguide 16 of Fig. 2. Other arrangements by which different effective dielectric constants el and e, can be derived are known to those skilled in the art.

- Thus the invention disclosed comprises a system wherein a pair of hollow pipe waveguides are of the same physical length, and in which the dielectric constant and transverse waveguide dimensions may be selected to make the rate of change of phase shifts with respect to Wavelength (or frequency) equal at the operating frequency. lA broad-banded effect is thereby secured, whereby the relative phase shift in the two waveguides is rmaintained over a large band of frequencies about the operating frequency.

What is claimed is:

1. `A microwave antenna system for maintaining substantially constant the relative phase shift of microwave energy transmitted or received over a broad range of frequencies about an operating frequency comprising, a first hollowpipe waveguide of physical length l, width al. height b, guide wavelength am, and an effective dielectric constant e, at said operating frequency, a second hollowpipe waveguide of physical length l, width a, height '1), guide wavelength ag, where 'A2, is different from am, and an effective dielectric constant e2 at said operating frequency where e, is different from el, el, AE1, et, and XZ, being related by th'e equation @pqr-egt@ said first and second waveguides being positioned side by side, means ycoupled to one end of said first and second waveguides for introducing 'microwave energy into said waveguides whereby the phase of the energy introduced into said first waveguide differs from the phase of the energy introduced into said second waveguide by an amount equal Vto the difference in phase shift of the energies transmitted through said waveguides, and a microwave horn connected to the other end of said rst and second waveguides for radiating a pair of overlapping beams of microwave energy.

2. Apparatus as claimed in claim 1 wherein a1=a2. 3. Apparatus as clarned in claim 1, said second waveguide having an elective dielectric constant of unity, and

a wavelength kg, at the operating frequency, said iirst waveguide having an effective dielectric constant e and a waveguide wavelength Agl at the operating frequency, e1, x21, and Ag, being related by the equation ekglIkgz.

4. Apparatus as clamed in claim 1, said waveguide being rectangular, and being proportioned for the transmission energy at the operating frequency in the dominant mode and having metallic walls and equal internal transverse dimensions between said walls, one of said waveguides having an eiiective dielectric constant substantially equal to unity, the other said waveguide having a dielectric of dielectric constant e, and

where xg, is the waveguide wavelength at said operating frequency for the solid dielectric waveguide and kg, is the waveguide wavelength at the operating frequency of the other said waveguide.

5. Apparatus as claimed in claim 1, said first and second waveguides having a common wall.

6. Apparatus as claimed in claim 1, said iirstl and second waveguides being straight along their lengths and disposed side-by-side with said ends to which energy is applied at substantially the same point, and the other ends of said waveguides at substantially another point.

7. Apparatus as claimed in claim 1, said rst waveguide being iilled with a single solid dielectric substance and said second waveguide being air-tilled.

8. Apparatus as claimed in claim 1, said first waveguide being only partially and not entirely lled withl a solid dielectric, and said second waveguide being filled with two different solid dielectrics.

References Cited in the file of this patent UNITED STATES PATENTS 2,129,669 Bowen Sept. 13, 1938 2,407,911 Tonks Sept. 17, 1946 2,464,269 Smith Mar. 15, 1949 2,513,498 Lawson July 4, 1950 2,741,744 Driscoll Apr. 10, 1956 2,765,401 Riblet Oct. 2, 1956 

